1. Field of the Invention
The invention relates to a process for the continuous preparation of mixtures, in particular crosslinkable materials based on organosilicon compounds, which is characterized by low mechanical loading of the material being mixed during mixing of organosilicon compounds and solids, in particular reinforcing fillers.
2. Background Art
Crosslinkable materials based on organosilicon compounds which can be stored in the absence of moisture and crosslink on admission of moisture at room temperature, so-called RTV1 sealing compounds, have long been known. They contain mainly organosilicon compounds having hydrolysable groups and reinforcing fillers such as finely divided silicas. These products are used in large amounts as pointing and sealing materials in the construction industry, where, in addition to good adhesion to various substrates, they must have especially two properties. These are firstly high resistance to deformation when used in the uncured state, and secondly, low resistance to tensile load in the cured state. The resistance to deformation during use, i.e. in particular the resistance during the smoothing process of the pointing, is also referred to by the user as “body”. This “body” must be as great as possible according to the requirements of the pointer. Although RTV1 sealing materials are characterized by good mechanical stabilities, i.e. already have a certain resistance to deformation in the uncured state, the user as a rule desires substantially greater resistance to deformation. The resistance of the cured RTV1 sealing compound in the joint under tensile load should generally be low, since this ensures that the action of force on the adhesive surface of the sealing compound with the substrate is as small as possible. If the resistance under tensile loads were high, strong forces would then act on the adhesive surfaces of the RTV1 sealing compound, for example if structure joints become larger as a result of cooling of components. Tearing away of the sealing compound from the substrate will then very quickly become possible and the sealed joint will no longer perform its function.
The two properties described can each be established very readily via the content of reinforcing filler in the RTV1 sealing compound. However, while the resistance to deformation in the uncured state requires high proportions of reinforcing filler in the RTV1 sealing compound, small amounts of reinforcing filler are required for achieving low tensile stress values of the cured RTV1 sealing compound. An upper limit for tensile stress values is obtained, for example, from the standard ISO 11600 Class 25 LM. This requires a tensile stress value at 100 percent elongation (modulus) of a so-called H test specimen of not more than 0.4 MPa. This standard can be complied with only if the proportion of reinforcing fillers in the RTV1 sealing compound is kept as small as possible. Higher proportions of reinforcing fillers rapidly result in a drastic increase in the modulus beyond the acceptable limit. However, no similar standard for ensuring maximum resistance to deformation on pointing exists to date. The known continuously prepared RTV1 sealing compounds therefore contain relatively low proportions of reinforcing fillers because these low proportions ensure compliance with the standard for a low modulus, mentioned above by way of example. The disadvantage of low resistance to deformation during use, which is not desired by the user, is generally accepted.
There have already been numerous attempts to increase the resistance to deformation during pointing, without exceeding the limit for maximum permissible tensile stress values. However, these solutions very frequently influenced other properties of the RTV1 sealing compound in a disadvantageous manner. Thus, there is in principle the possibility of using high polymer viscosities. This would both increase the resistance to deformation during pointing and permit a low tensile stress value on a tensile load. However, this is subject to limits in that such extremely high polymer viscosities can no longer be handled technically. Moreover, the RTV1 sealing compounds would then have undesirably high stringiness during use.
EP-A1-857 750 proposes also mixing in polyalkylene oxides as rheology additives. However, these additives have a very disadvantageous effect on the required adhesion of the RTV1 sealing compound on various substrates. The field of use of the crosslinkable material is therefore very limited. The processes described to date for the continuous preparation of RTV1 sealing compounds present a further problem. In spite of the relatively small amounts of the reinforcing fillers which have to be mixed into the organosilicon compound, the continuous preparation of RTV1 compositions is difficult since the ingredients are pulverulent solids and highly viscous polymers which are very difficult to mix with one another.
The production of RTV materials is therefore effected in mixing units having very high-speed mixing members. Owing to the high speeds, a very large amount of reinforcing filler is to be mixed into the organosilicon compound in a short time. Thereafter, the high speeds are used for achieving as uniform a dispersion as is possible with the reinforcing filler. EP-A-1 008 613 describes, for example, a mixing turbine. By means of the high speeds of the rotor and small sheer gaps between rotor and wall, the mixed material is subjected to mechanical load so that the temperature of the mixture increases to above 100° C. EP-B-0 512 730 teaches the incorporation of finely divided silica into organopolysiloxanes with the aid of a twin-screw kneader. Here too, there is a considerable temperature increase of the material being mixed, owing to the high mechanical load during the mixing process. Independently of the mechanical load, the thermal load can also lead to damage to the RTV materials. Thus, for example, the shelf-life may be limited.
All described processes for the preparation of RTV materials additionally have, as a rule, the considerable disadvantage that extremely highly viscous mixed material has to be cooled again in a technologically very complicated manner after mixing, so that it can be introduced into moisture-tight containers, such as drums, cartridges or tubular bags. If the mixed material were too warm, the container would deform owing to the volume shrinkage after the cooling of the RTV1 sealing compound.